1. Field of the Invention
The invention relates to an apparatus and a method for implanting ions on a target, and more particularly relates to the apparatus and the method for preventing impurity ions, which are different from desired ions in at least one of mass number and energy, from being implanted on a target.
2. Description of the Related Art
FIG. 2 shows an example of an ion implantation apparatus of the related art. The ion implantation apparatus includes an ion source 2 having a plasma production chamber 4 for electrostatically extracting ions 8 therefrom, a mass segregation magnet 10 for selectively deriving specific ions (which are specified by a mass number and valence) from the ions 8 extracted from the ion source 2, an acceleration pipe 12 for electrostatically accelerating the ions 8 derived from the mass segregation magnet 10, and a momentum segregation magnet 14 for selectively deriving the ions having a specific momentum (which is specified by a mass number and energy) from the ions 8 derived from the acceleration pipe 12. The ion implantation apparatus is constructed so that only the desired ions derived from the momentum segregation magnet 14 are implanted on a target (e.g. substrate such as a semiconductor wafer) 16.
The target 16 is held on a holder 18 at a ground potential. A scanner for scanning ions 8 and others not shown are usually provided between the momentum segregation magnet 14 and the target 16. A path of the ions 8 from the outlet of the ion source 2 to the holder 18 is included in a vacuum chamber. The vacuum chamber is not shown.
The ion source 2 includes the plasma production chamber 4 for generating plasma and an extraction electrode 6 for extracting the ions 8. The plasma production chamber has a positive potential, and then an extraction voltage VE applied therebetween from a DC extraction electrode 20. The ions 8 are electrostatically generated from the plasma production chamber 4 by the extraction voltage.
The acceleration pipe 12 has a plurality of electrodes 13. The acceleration pipe 12 has a positive potential, and then a voltage VA is applied between an inlet and outlet electrode 13 from a DC accelerating power source 22. The ions 8 are electrostatically accelerated by the accelerating voltage VA so that the ions 8 has target energy.
Assuming that the valence of the desired ions is ZI, the total energy ET of the desired ions incident on the target 16 is expressed by the following equation.
ET=(VE+VA)xc3x97ZI[eV]xe2x80x83xe2x80x83[Equation 1]
Since the following phenomenon occurs between the outlet of the mass segregation magnet 10 and the inlet of the acceleration pipe 12, impurity ions which are different from the desired ions may be mixed into the desired ions in the ion implantation apparatus.
(1) The energy accelerated by the acceleration pipe 12 is changed by charging conversion in which the desired ions collide with a residual gas. For example, when doubly charged ions are converted into singly charged ions by the charging conversion, the energy accelerated by the acceleration pipe 12 becomes half of that in the case of the doubly charged ions, if the Voltage VA is constant.
(2) Where the desired ions are molecular ions, by molecular dissociation, the desired ions change into different ions. For example, when BF2 ions dissociate into BF ions and F ions, or B ions and F ions, the BF2 ions no longer are the desired ions.
(3) A part of the ions 8 collides with the member which constitutes an apparatus such as the vacuum chamber so that atoms or molecules of the member are out of the surface of the member by a sputtering to become impurity ions.
(4) A part of the ions 8 collides with the member which constitutes an apparatus such as the vacuum chamber so that atoms or molecules deposited on or implanted in the member during previous operation of the ion implantation apparatus are out of the member surface by sputtering to become impurity ions.
(5) The gas or vapor used to generating plasma in the plasma production chamber 4 of the ion source 2 flows out from the plasma production chamber 4 into a passage of the ions 8, and then the flown gas or vapor is ionized on the passage to the inlet of the acceleration pipe 12, or otherwise the flown gas or vapor reacts with the atoms or molecules generated owing to the phenomena of the above items (3) and (4) thereby to become impurity ions.
In the ion implantation apparatus, it is not desired that the impurity ions which are different from the desired ions in at least one of mass number and energy are implanted into the target 12 together with the desired ions. Accordingly, a desired implantation characteristic of the target cannot be obtained.
Therefore, when ion implantation with high purity is required as in the example shown in FIG. 2, the momentum segregation magnet 14 as described above as well as the mass number segregation magnet 10 is provided behind the acceleration pipe 12 in order to derive only the ions having a specific momentum selectively.
The momentum segregation magnet 14 permits the impurity ions generated by the phenomena of the above items (1) and (2) to be removed. The impurity ions having a different momentum from that of the desired ions are generated in the phenomena of (1) and (2).
However, in case the impurity ions are generated by the phenomena of the above items (3) to (5), the impurity ions which satisfy the following Equation 2 cannot be separated and removed from the desired ions by means of the momentum segregation magnet 14. This applies to the case where the left side≈the right side in Equation 2 (that means, the left side of the Equation 2 is equal or about equal to the right side thereof). Now it is assumed that MT denotes the mass number of the desired ions, ZI denotes the valence thereof, MC denotes the mass number of the impurity ions at issue, and ZC denotes the valence thereof. VE and VA have been already defined.
MIxc2x7(VE+VA)/ZI=MCxc2x7VA/ZC 
Assuming that B is a magnetic flux density, VT is an entire acceleration voltage, m is a mass, and q is a charge, the circling radius R of the ions in the momentum segregation magnet 14 is expressed by a following Equation 3. Now, assuming that M is the mass number of the ions and mP is the mass of a proton, m=Mxc2x7mP. Further, Z is the valence of the ions and e is an electron weight, q=Zxc2x7e. In short, Equation 3 implies that the ions with the same Mxc2x7VT/Z provides the same circling radius R.
R=Bxe2x88x921{square root over ((2mVT/q))}xe2x80x83xe2x80x83[Equation 3]
Therefore, if the above Equation 2 is satisfied, in the momentum segregation magnet 14, the circling radii of the desired ions and impurity ions are equal to each other, both cannot be separated from each other. As a result, even with the momentum segregation magnet 14, the impurity ions as well as the desired ions are implanted into the target 16. This applies to the case where the left side≅the right side of the Equation 2 (that means, the left side of the Equation 2 is equal or about equal to the right side thereof).
A case where the valence ZI of the desired ions and the valence ZC of the impurity ions at issue are equal to each other (i.e. where ZI=ZC) is typical. For example, both are singly charged ions. In this case, the above Equation 2 can be represented by the following equation.
MIxc2x7(VE+VA)=MCxc2x7VIxe2x80x83xe2x80x83[Equation 4]
If the ion implantation is carried out for the target 16 on the condition of the Equation 4, the desired ions as well as the impurity ions will be implanted into the target 16. This applies to the case where the left side≅the right side (that means, the left side of the Equation 2 is equal or about equal to the right side thereof). Such implantation is not preferable.
It can be supposed that the momentum segregation magnet 14 is served as the mass segregation magnet without providing the mass segregation magnet 10. However, in such a case, the above problem becomes more serious. In case the mass segregation magnet 10 is not present, the impurity ions are generated between the outlet (more particularly, outlet of the extraction electrode 6) and the inlet of the acceleration pipe 12 in this wider range than the above range.
It is an object of the invention to prevent impurity ions, which are different from desired ions in at least one of mass number and energy from being implanted into a target together with the desired ions.
The method for implanting the desired ion on the target according to this invention is characterized in that if the relationship of Equation 2 is satisfied or in Equation 2, the left side≅right side, one of the extraction voltage VE and the acceleration voltage VA is increased and the other thereof is decreased while the value of (VE+VA) is substantially constant.
The apparatus for implanting the ion on the target according to the invention is characterized in that it includes a control device for increasing one of the extraction voltage VE and the acceleration voltage VA and decreasing the other thereof while maintaining the value of (VE+VA) substantially constant and satisfying the Equation 2.
When a voltage to be increased or decreased is represented by xcex94V, the above Equation 2 after voltage adjustment is converted into the following Equation 5 or Equation 6. Either method of Equation 5 and Equation 6 may be adopted.
MIxc2x7{(VExe2x88x92xcex94V)+(VA+xcex94V)}/ZIxe2x89xa0MCxc2x7(VA+xcex94V)/ZCxe2x80x83xe2x80x83[Equation 6]
MIxc2x7{(VExe2x88x92xcex94V)+(VAxe2x88x92xcex94V)}/ZIxe2x89xa0MCxc2x7(VAxe2x88x92xcex94V)/ZCxe2x80x83xe2x80x83[Equation 6]
In both cases of Equation 5 and Equation 6, the value of the left side and that of the right side are not equal to each other. In this way, as understood from the explanation of the above Equation 3, in the momentum segregation magnet, since the circling radius of the desired ions and that of the noted impurity ions are made different from each other, the impurity ions can be removed by the momentum segregation magnet to derive selectively only the desired ions which are implanted into the target. Namely, it is possible to prevent impurity ions which are different from the desired ions in at least one of their mass number and energy from being implant*ed into the target together with the desired ions.
Further, when the above voltage is adjusted, since the value of (VE+VA) which is a sum of the extraction voltage VE and the acceleration voltage VA is kept substantially constant, it is not necessary to vary the total energy of the desired ions on the target. For this reason, the initial ion implanting condition of the desired ions can be maintained.